The present invention relates to a method of manufacturing a blank for a light-weight mirror with a supporting structure that has cells and that can be fastened securely to a mirror plate or backing plate, whereby the materials selected for the supporting structure and for the mirror plate or backing plate have practically the same coefficient of thermal expansion and whereby the supporting structure is manufactured by positioning mutually separated graphite molding bodies on a base in the form of a plate, surrounding them with a wall of graphite, filling the interstices between the molding bodies and between the molding bodies and the wall with a granulate of crystalline or amorphous quartz to create part of the supporting structure, placing the overall assembly in a furnace where it is baked in a non-oxidizing atmosphere, and removing the graphite molding bodies once the piece has cooled.
U.S. Pat. No. 4 466 700 discloses a method of manufacturing a light-weight mirror, especially for applications in astronomy. A prefabricated supporting structure of quartz glass, fused quartz, or high-silicate glass is positioned between a front plate made out of quartz glass or high-silicate glass and a backing plate of quartz glass, fused quartz, or high-silicate glass and securely fastened to the plates. The materials selected for the plates and supporting structure have practically the same coefficient of thermal expansion. The supporting structure is created by placing plate-shaped and/or tubular pieces of quartz glass, fused quartz, or high-silicate glass on a supporting plate coated with a material, graphite for example, that contains carbon, filling the interstices between each pair of adjacent pieces with a sintering mass of granulate, small tubelets, rods, or flakes, or a mixture thereof, surrounding the assembly with an annular wall of graphite, and baking it in a furnace in a non-oxidizing atmosphere for 2 to 6 hours at a temperature of from 1300.degree. to 1600.degree. C. The plate and wall are then removed once the piece has cooled. Molding bodies made out o a material that contains carbon or graphite can also be positioned on the supporting plate and removed along with it. The baking sinters the particles of quartz not only together but also to the tubular pieces. Once the graphite pieces have been removed, the supporting structure is fastened to the front plate and backing plate by oriented thermal action such as arc fusing for example.
The method disclosed in that patent accordingly necessitates positioning a number of prefabricated tubular pieces of quartz material on the supporting plate and packing them with granulated material in order to obtain the desired honeycomb structure subsequent to sintering. Since the space taken up by the granulate can be considerably reduced by introducing graphite molding bodies, only a comparatively small amount of space needs to be filled to attain high stability. Still, the method is relatively expensive. The tubes of quartz material that function as a starting material must be prefabricated in a series of stages, assembled onto a base for the supporting structure, the interstices filled with granulate, and sintered together into a structure. Furthermore, arc fusing the supporting structure to the front plate and backing plate involves still a third step, and the resulting incomplete heating can cause considerable stress to the detriment of stability.